1. Field of the Invention
The present invention relates to a drum type core for use in a high frequency coil, and the like, more particularly to a drum type core that, without using a bobbin at its winding portion, enhances insulation performance and protects coil wires.
2. Description of the Related Art
A ferrite drum type core, which comprises: a central cylinder portion having a small diameter and having a coil provided therearound; and two flange portions having a large diameter and provided at respective ends of the cylinder, has been universally known and is widely used in high frequency transformers, high frequency coils, and the like. The conventional drum type core is structured usually such that the central cylinder portion is integrated with the flange portions formed as a single piece.
The drum type core thus structured can not be completed by press molding alone but is produced such that at first, a ferrite is molded into a cylinder, then the central portion of the cylinder is circumferentially machined by centerless grinding. Due to the configurational constraint of a grinding wheel employed, the flange portions have right-angled inside edges.
Wire terminals of the coil provided around the cylinder portion are usually bound around and soldered to terminal pins attached to a core or a base. In the conventional drum type core, it can happen that the wire terminals touch the edges at the inside faces of the flange portions and have their insulation coating damaged or peeled off.
Especially when a Mn—Zn ferrite is used as a core material, a slightest damage on the wire can lead to a short-circuit problem due to the material having a very low surface electrical resistance. The Mn—Zn ferrite has distinctive electrical characteristics that can not be obtained with a Ni—Zn ferrite conventionally used in a high frequency range, but has the above described critical drawback of poor insulation performance.
To improve the insulation performance, the core is coated with varnish, but the edges of the flange portions are hard to be sufficiently covered, which makes the insulation imperfect, resulting in failure to prevent the short-circuit from happening. To overcome the problem of the conventional art, a drum type core, which is made of a material with a low surface electrical resistance such as a Mn—Zn ferrite and is still free from the above described wire insulation failure, is disclosed in Japanese Utility Model Patent No. Hei 6-35445.
The drum type core disclosed therein is advantageous especially when the core is made of a material with a low surface electrical resistance, such a Mn—Zn ferrite. In the drum type core, a coil is provided on a resin bobbin, and wire terminals of the coil are taken out along a cranked extension from the flange portion of the core and usually are bound around and soldered to terminal pins disposed outside.
FIGS. 9 and 10 are respectively an exploded perspective view and a cross-sectional view of an embodiment of the above described conventional drum type core. Referring to FIG. 9, the drum type core comprises: a single-flanged cylinder 114 composed of a cylinder portion 110 and a flange portion 112 formed integrally with the cylinder portion 110; and a counterpart flange piece 118, into whose central hole a plain end of the flanged cylinder 114 is to be inserted. A resin bobbin 116 configured uniquely is provided around the cylinder portion 110.
The flanged cylinder 114 and the counterpart flange piece 118 are sintered compacts made of, for example, a Mn—Zn ferrite. Unlike conventionally, these two components are produced separately and so can be completed easily by press molding, thereby eliminating the process of centerless grinding
The resin bobbin 116 is molded of a synthetic resin as a single piece with a very small wall thickness, and includes two flanges 116a, 116a which have an outer diameter equal to or slightly larger than that of the flange portion 112 or the counterpart flange piece 118. The bobbin 116 includes further two flange extensions 116b, 116b configured annular and extending outward respectively from and perpendicular to the bobbin flanges 116a, 116a so as to cover respective inside edges of the flange portion 114 and the counterpart flange piece 118.
Referring to FIG. 10, the bobbin 116 is provided with a coil 120 wound thereon, is then fitted onto the cylinder portion 110, and the counterpart flange piece 118 is attached to the plain end of the cylinder portion 110 and fixed using adhesive or the like, thus completing the drum type core.
In the above described drum type core, the wire terminals of the coil are protected by the bobbin thereby preventing insulation failure even when the core, is made of a material with a low surface electrical resistance such as a Mn—Zn ferrite. The drum type core, however, requires a bobbin, which pushes up the product cost, and also reduces the coil space, namely space factor, rendering the above described art unsuitable, especially for a small size drum type core.